Mobile communication is one of the most important technologies for contributing to social and economic development around the world. Optimizing energy efficiency will not only reduce environmental impact, it will also cut network costs which will give benefits for all using the mobile systems.
Modern standards as WCDMA, LTE and WiMAX have very high capacity in terms of users and throughput, which requires a large amount of energy. In order to achieve high data throughput in the cellular systems a dense cell plan has to be deployed. A base station consumes considerable amount of power, typical 65000 kWh per station and year.
Network design is a key issue improving the energy-efficiency. No amount of energy efficiency at the component level can make up for an inefficiently designed network. For instance the number of radio sites should be optimized for the coverage and quality that needs to be achieved.
In order to achieve an energy-efficient design a number of issues have to be addressed from start. At first, the true network needs has to be addressed. The exact coverage, capacity and quality have to be considered before getting into considerations about individual sites and equipment specifications. Moreover, the current and future business environment needs has to be considered, considering the possibility to rebuild or expand sites. Once these factors have been considered the operator should begin the network design process, looking into the total cost of the ownership and the alternative design options.
Capital expenditure typically represents a very small portion of the total cost of the ownership. Instead, the long term savings from site reduction and efficient operation is significant, with a significant reduction in energy consumption as a key issue.
Optimizing solutions for reducing energy consumption means that every stone has to be turned over. Still, the total network solution is greater than the sum of their parts. This means that combining the best components in a package does not always give the best results. In the radio base station the relative energy consumption of the different components vary on the dependency of the properties of the components it has to work with.
Typical sources of energy consumption in the base station are signal processing, RF conversion, power amplification, power supply, climate equipment (air conditioning) and feeder. For instance, In traditional base stations the equipment is located on the ground which means that the antennas has to be fed using several meters of cable. Half of the emitted power can be lost in the feeders. By placing the equipment in the top of the tower, significant reductions in energy consumption is achieved. The equipment can be combined with a battery back-up unit that minimizes hardware and energy consumption.
Another way in which energy reduction can be achieved is through the use of stand-by modes. Base station sites are dimensioned to cope with peak hours. In a cell a number of TRX (transmitters) can run at the same time. Using power management schemes, some TRX can be put in stand-by instead of running in idling mode during low traffic hours.
Other ways of reducing the energy consumption is to avoid unnecessary DC/DC conversion and reduce the need of cooling fans and cooling systems. Modules based on digital power management can also reduce energy consumption.
There is an increasing need of delivering wireless technology with broadband capacity for cellular networks. A good broadband system must fulfill certain criteria, such as high data rate and capacity, low cost per bit, good Quality of Service and greater coverage. High Speed Packet Access (HSPA) and Mobile WiMAX are examples two network access technologies that enable this. Both of these uses a frame structure for the uplink and downlink communication between the base station and the mobile terminal. In the following part of the background, the technology of WiMAX will be introduced as an example of a technology using frame structure, but other technologies such as WCDMA, GSM, HSPA and Long Term Evolution (LTE) also use frame structuring. The frames of the different technologies differ to some extent.
WiMAX refers to the IEEE standard 802-16 where Mobile WiMAX relates to 802.16e-2005. Mobile WiMAX is an improvement of the modulation schemes used in earlier (fixed) WiMAX standards by the introduction of Scalable Orthogonal Frequency Division Multiple Access (SOFDMA) to carry data and supporting channel bandwidths with a large number of sub-carriers on different frequencies (sub-channels) within the band. The large number of sub-carriers improves the performance in multipath fading channels.
Scalable OFDMA is a statistical multiplexing technology, and scalable refers to the ability of the communication channel to be divided into a number of variable bit-rate digital channels (sub-carriers) or data streams. It means a dynamic scheduling wherein a time slot in the access assigned by the base station can enlarge and contract but still remain assigned to the particular mobile terminal. Different numbers of sub-carriers can be assigned to different users, and the Quality of Service, i.e. data rate and error probability, can be controlled individually for each user since the sub-channels are variable. The band-width of the channel can flex between 1.25 and 20 MHz. OFDMA (on which SOFDMA is based) has fixed sub-carrier band-width.
OFDMA is a multi-user version of Orthogonal Frequency Division Multiplexing (OFDM) modulation scheme. OFDM is for one single user in contrast to OFDMA. OFDM(A) uses a large number of sub-carriers, in which each sub-carrier is modulated for instance with Quadrature Amplitude Modulation (QAM). OFDM has the ability to cope with severe channel conditions, which makes Mobile WiMAX very robust. OFDM also has high spectral efficiency. OFDM may be viewed as using many slowly-modulated narrowband signals rather than one rapidly-modulated wideband signal. QAM will not be described any further in this document.
The duplex method of Mobile WiMAX is Time Division Duplex (TDD). TDD only occupies one single channel, with uplink and down link traffic assigned to different time slots. TDD with OFDMA provides subchannels and time slots enabling multi access for different users. TDD has an advantage in the case where the asymmetry of the uplink and downlink data speed is variable. As the amount of uplink data increases, more bandwidth can dynamically be allocated to that.
The ability of sub-channelling by OFDMA is shown in FIG. 1, which figure illustrates the frame structure schematically. The frame structure as visualized comprises a number of subchannels and a number of time slots, enabled by OFDMA being a statistical multiplexing technique. The data regions 11,12,13,14 of the different user devices 11,12,13,14 are illustrated in the figure.
Mobile WiMAX transmitted via base stations uses SOFDMA with TDD. FIG. 2 shows a more detailed schematic view of a frame structure for OFDMA when operating in TDD mode. The frame (Frame N) comprises a downlink subframe 15, a following uplink subframe 16, a small guard interval 20 between the downlink and uplink subframe and an end interval 22 between the uplink and the downlink subframe of the next frame. In mobile WiMAX these frames are 5 ms long. Some WiMAX systems support OFDMA operating in Frequency division duplexing (FDD) in which the frame structure differs from TDD in that the uplink and downlink frames are transmitted at the same time over different carriers. TDD will in the future be used for most WiMAX deployments, since it allows for a more flexible sharing of bandwidth between up- and downlink, does not requires paired spectrum and has a reciprocal channel that can be exploited for spatial processing.
The downlink subframe 15 in TDD begins with overhead information for informing the user device about the characteristics of the system. The overhead comprises synchronization information 17 and system information 18. The overhead is followed by data regions 19 for the downlink data traffic in the downlink subframe. A guard interval 20 is followed by an uplink subframe 21 with data regions for the uplink data traffic from the different user devices. Finally there is the end interval 22 followed by the overhead synchronization information 17 of the next frame.
In WiMAX particularly, the overhead begins with a downlink preamble that is used for physical-layer procedures (cell detection, time and frequency synchronization). The preamble is followed by a frame control header providing frame configuration and system information (modulation and coding maps) to find where and how to decode downlink and uplink. The frame control header and maps are sent for each available data region 19, 21.
Uplink and downlink subframes can instead of TDD be divided with Frequency Division Duplex. FDD is more efficient in the case of symmetric traffic. Another advantage is that it makes radio planning easier and more efficient. Compared with TDD, FDD divides the subframe by frequency instead, which means that the subframes are sent at the same time using different frequencies.
In order to achieve high data throughput in cellular systems, high order modulation, e.g. 64 QAM and high transmit power is used at the base station. The physical resources in term of subcarriers and time are kept to a minimum to maximize the user data throughput. High performance power amplifier is needed to keep the signal prosperities after the amplification. Especially the linearity of the amplification is important. This requires a lot of energy which increases the energy consumption of the base station. Due to these requirements the amplifier efficiency is low and contributes to a large extent the base station power consumption.
During low load or no load scenarios the base station still needs to transmit the system and synchronization information 17,18 to serve the attached mobiles and so a new mobile can access the system. The information has to be transmitted with enough power to reach all mobiles within the cell and is therefore transmitted with low modulation order and high output power. Due to these transmissions the base station power consumption is still quite significant.